Nicholas Weaver and Vern Paxson have published a paper estimating the
worst case scenario of a network worm attack from USD$52 to $103 Billion.
http://www.icir.org/vern/papers/worst-case-worm.WEIS04.pdf
Although it was published last month, it was quoted in a new article
today. One thing the left out the study was the improvement in
productivity that occurs after you rebuild your Microsoft Windows
computer. Some people regularly rebuild their Windows computer a few
times a year. If companies and people have already factored in the
need to regularly rebuild Windows computers in the Total Cost of
Ownership, would a network worm really have that much additional
economic impact? What's the economic impact of CTRL-ALT-DEL? The
marketplace has already factored it in, because its business as
usual.
The well known (but not necessarily well respected) Mi2g Intelligence Unit
has estimated the costs for previous infections as follows:
Sobig: $37.1 billion
MyDoom: $22.6 billion
Klez: $19.8 billion
Mimail: $11.5 billion
Yaha: $11.5 billion
Swen: $10.4 billion
Love Bug: $8.8 billion
Bugbear: $3.9 billion
Dumaru: $3.8 billion
SirCam: $3 billion